Known in the art is a spark ignition type internal combustion engine provided with a variable compression ratio mechanism able to change a mechanical compression ratio and a variable valve timing mechanism able to control a closing timing of an intake valve, performing a supercharging action by a supercharger at the time of engine medium load operation and engine high load operation, and increasing the mechanical compression ratio and delaying the closing timing of the intake valve as the engine load becomes lower at the time of engine medium and high load operation in the state holding the actual combustion ratio constant (for example, see Japanese Patent Publication (A) No. 2004-218522).
However, as will be explained in detail later, it has been found that in such internal combustion engines, the air-fuel ratio at which the thermal efficiency becomes high differs according to the mechanical compression ratio, accordingly, to raise the thermal efficiency, it is necessary to appropriately select an air-fuel ratio according to the mechanical compression ratio. However, the above-mentioned document does not allude at all to this.